1. Field of the Invention
The present invention relates to a lens-interchangeable image pickup apparatus provided with a focus detecting function.
2. Description of the Related Art
A technique called a contrast type method (a hill-climbing method or a TV-AF method) has been employed for AF (autofocus). In the contrast type method, the contrast of a picked-up image obtained in each driving stage is acquired as an evaluation value while driving a focus lens included in a photographic lens or an image pickup element in an optical axis direction. Then, a lens position corresponding to the highest evaluation value is defined as an in-focus position. The contrast type AF is described in detail e.g. in Japanese Patent No. 2821214.
In the contrast type AF, evaluation values each calculated from high-frequency components of an object image are acquired at a plurality of focus adjustment positions, respectively, whereby an in-focus position is determined. At this time, the method of driving a lens for shifting the focus adjustment position includes a method of temporarily stopping the driving of the lens at each target position for acquiring an evaluation value (see e.g. Japanese Patent Laid-Open Publication No. H06-141223) and a method of acquiring evaluation values while continuing the driving of the lens (see e.g. Japanese Patent Laid-Open Publication No. 2002-72073). Hereinafter, the former method will be referred to as “the step driving method”, and the latter as “the search driving method”.
In the step driving method, an evaluation value is acquired at each drive stop position while intermittently performing lens driving by a relatively small defocus amount. The step driving method is advantageous in that focus adjustment positions where a lens is to be stopped at completion of lens driving are predetermined, and hence an evaluation value can be reliably acquired at each predetermined focus adjustment position. However, lens driving by a small defocus amount requires a time period for acceleration or deceleration occurs, and therefore it takes longer time before completion of lens driving than in a case where lens driving is performed by a fixed defocus amount at a fixed speed.
In the search driving method, evaluation values are each acquired at predetermined intervals of time dependent on the charge storage interval of a sensor for use in focus detection while continuously performing lens driving to thereby continuously change the focus adjustment position. The search driving method is advantageous in that the acceleration or deceleration of lens driving is not sharp except at the start and end of the lens driving, which makes shorter the time required for lens driving between two evaluation value acquisition positions than in the step driving method. However, assuming that the charge storage interval is fixed, an evaluation value acquisition distance interval increases with an increase in the lens driving speed.
FIGS. 6A to 6E are schematic diagrams for comparison of evaluation value acquisition timings and detected evaluation values between the step driving method and the search driving method.
FIG. 6A shows an example of an output level of an evaluation value of an object which varies with the focus adjustment position. Reference numeral 601 denotes a waveform representing the output of the evaluation value. The horizontal axis represents the focus adjustment position, and the vertical axis represents the level of a contrast evaluation value dependent on the focus adjustment position. Reference numeral 602 denotes an auxiliary line showing the output peak level of the evaluation value.
FIG. 6B shows evaluation value acquisition timings and evaluation values to be detected in a case where the focus lens is search-driven at a speed V1. Reference numeral 603 denotes a defocus amount by which the focus lens is advanced per unit time during search driving performed at the speed V1. Arrows 604, 605, and 606 indicate the respective evaluation value acquisition timings in the search driving performed at the speed V1. Reference numeral 607 denotes changes in the evaluation value to be detected during the search driving performed at the speed V1 in a case where the charge storage time period of the sensor for focus detection is set to be short and electric charge is stored at a focus adjustment position immediately before the associated evaluation value acquisition timing. Reference numeral 608 denotes an auxiliary line indicating the output peak level of the evaluation value 607. Reference numeral 609 denotes changes in the evaluation value to be detected during the search driving performed at the speed V1 in a case where the charge storage time period of the sensor for focus detection is set to a length approximately equal to an evaluation acquisition time interval and an approximately intermediate point between two adjacent ones of the evaluation value acquisition timings corresponds to the center of gravity in terms of charge storage in focusing. Reference numeral 610 denotes an auxiliary line indicating the output peak level of the evaluation value 609. It should be noted that in FIG. 6B, a broken line shows the waveform 601 representing the output of the evaluation value. As shown in FIG. 6B, as the charge storage time period becomes shorter, values which are more instantaneous in changes in evaluation values are detected. Further, when the focus lens is search-driven at the speed V1, the evaluation value acquisition distance interval is increased, and hence an evaluation value considerably lower than the output peak level 602 of the contrast evaluation value of an object is more likely to be detected.
FIG. 6C shows evaluation value acquisition timings and evaluation values to be detected in a case where the focus lens is search-driven at a speed V2 lower than the speed V1. Reference numeral 611 denotes a defocus amount by which the focus lens is advanced per unit time during search driving performed at the speed V2. Arrows 612, 613, and 614 indicate the respective evaluation value acquisition timings in the search driving performed at the speed V2. Reference numeral 615 denotes changes in the evaluation value to be detected during the search driving performed at the speed V2 in a case where the charge storage time period of the sensor for focus detection is set to be short and electric charge is stored at a focus adjustment position immediately before the associated evaluation value acquisition timing. Further, reference numeral 616 denotes an auxiliary line indicating the output peak level of the evaluation value 615. Reference numeral 617 denotes changes in the evaluation value to be detected during the search driving performed at the speed V2 in a case where the charge storage time period of the sensor for focus detection is set to a length approximately equal to an evaluation acquisition time interval and an approximately intermediate point between two adjacent ones of the evaluation value acquisition timings corresponds to the center of gravity in terms of charge storage in focusing. Reference numeral 618 denotes an auxiliary line indicating the output peak level of the evaluation value 617. It should be noted that in FIG. 6C, a broken line shows the waveform 601 representing the output of the evaluation value. As shown in FIG. 6C, as search driving speed is lower, the evaluation value acquisition interval on a focus axis becomes shorter, which makes it possible to more easily detect an evaluation value close to the output peak level 602 of the contrast evaluation value of an object.
FIG. 6D shows evaluation value acquisition timings and evaluation values to be detected in a case where the focus lens is search-driven at a speed V3 still lower than the speed V2. Reference numeral 619 denotes a defocus amount by which the focus lens is advanced per unit time during search driving performed at the speed V3. Arrows 620, 621, and 622 indicate the respective evaluation value acquisition timings in the search driving performed at the speed V3. Reference numeral 623 denotes changes in the evaluation value to be detected during the search driving performed at the speed V3 in a case where the charge storage time period of the sensor for focus detection is set to be short and electric charge is stored at a focus adjustment position immediately before the associated evaluation value acquisition timing. Reference numeral 624 denotes an auxiliary line indicating the output peak level of the evaluation value 623. Reference numeral 625 denotes changes in the evaluation value to be detected during the search driving performed at the speed V3 in a case where the charge storage time period of the sensor for focus detection is set to a length approximately equal to an evaluation acquisition time interval and an approximately intermediate point between two adjacent ones of the evaluation value acquisition timings corresponds to the center of gravity in terms of charge storage in focusing. Reference numeral 626 denotes an auxiliary line indicating the output peak level of the evaluation value 625. It should be noted that in FIG. 6D, a broken line shows the waveform 601 representing the output of the evaluation value. As shown in FIG. 6D, when search driving speed is further lower, if the charge storage time period is short, the evaluation value level close to the output peak level 602 of the contrast evaluation value of an object can be detected more easily as an instantaneous value of the changing evaluation value.
FIG. 6E shows evaluation value acquisition timings and evaluation values to be detected in a case where the focus lens is step-driven. In FIG. 6E, the amount of step driving is depicted such that it coincides with the defocus amount by which the focus lens is advanced over a time period corresponding to the evaluation value acquisition time interval during search driving performed at the speed V3. Reference numeral 627 denotes a defocus amount by which the focus lens is advanced by one-time step driving. Arrows 628, 629, and 630 indicate the respective evaluation value acquisition timings in the step driving, and corresponds to respective stop positions of the focus lens in the step driving. Reference numeral 631 denotes changes in the evaluation value to be detected during the step driving in a case where the charge storage time period of the sensor for focus detection is set to be short and electric charge is stored at a focus adjustment position immediately before the associated evaluation value acquisition timing. Reference numeral 632 denotes an auxiliary line indicating the output peak level of the evaluation value 631. Reference numeral 633 denotes changes in the evaluation value to be detected during the step driving in a case where the charge storage time period of the sensor for focus detection is set to a length approximately equal to an evaluation acquisition time interval and an approximately intermediate point between two adjacent ones of the evaluation value acquisition timings corresponds to the center of gravity in terms of charge storage in focusing. Reference numeral 634 denotes an auxiliary line indicating the output peak level of the evaluation value 633. It should be noted that in FIG. 6E, a broken line shows the waveform 601 representing the output of the evaluation value. As shown in FIG. 6E, in the case of the step driving, each evaluation value is acquired after completion of lens driving, and hence the same evaluation value is detected irrespective of the length of the charge storage time period. Further, in the step driving, a charge storage result is acquired at the position where lens driving is completed, so that the focus lens is at rest at a position where lens driving is completed, during the charge storage time period. Therefore, the same evaluation value can be obtained irrespective of the length of the charge storage time period.
As described above, in the case of acquiring evaluation values while continuously driving the focus lens by the search driving method, the detected evaluation value level changes according to the speed of driving of the focus lens and the charge storage time period of the sensor for focus detection. On the other hand, in the case of acquiring evaluation values by the step driving method, i.e. acquiring an evaluation value each time lens driving is stopped, each detected evaluation value level changes according to the amount of step driving.
In a case where the lens-interchangeable image pickup apparatus performs the contrast type AF by the search driving method, the speed of driving of the focus lens becomes too high depending on a mounted interchangeable lens, so that it sometimes occurs that an evaluation value cannot be acquired at a target position. When the speed of driving of the focus lens is too high, causing an increase in the evaluation value acquisition distance interval, it becomes impossible to acquire an evaluation value at a focus adjustment position where high contrasts around an in-focus position can be obtained, which makes it impossible to detect the in-focus position properly. In this case, the output peak level of the evaluation value is detected to be low, which can degrade the accuracy of autofocus due to erroneous determination of an in-focus position. Further, there is a fear that the focus lens can continue searching for an in-focus position even after having passed the in-focus position, which causes an increase in autofocus time.